Optical components of transparent material, for example, optical ophthalmic lenses, must be tested for defects prior to their utilization, particularly for surfaces flaws such as scratches, smears, cracks, chips, stains, and for occlusions such as bubbles or streaks. Limit values of tolerable flaws are established in internationally recognized standards for precision optics, specifically DIN 3140 or MIL-0-13830 or less stringent standards used by the ophthalmic industry, such as DIN 58203.
Conventionally, the testing is carried out by visual inspection by trained personnel, mostly in a darkened room. Such a procedure is expensive, not sufficiently objective and not sufficiently reliable. Therefore, efforts are being made to develop methods and devices for the automatic and objective testing of optical components.
DE-OS 2337597 discloses a test method according to which a light ray is focused on the surface of the optical component to be tested and is sequentially moved over said surface, while being kept in focus. Illumination of the object to be tested is, in this case, achieved in that the test piece rotates on its axis and the impinging light ray is slowly radially deflected in such a way that it describes a spiral-shaped pattern on the test piece, whereby its state of focus must be continuously readjusted according to the curvature of the surface to be scanned. The light which penetrates the component is reflected backwards, passes once again through the component, and then impinges on a detector. The deviations in the intensity of the signal received make it possible to deduce a flaw and to localize it. Such a method, however, requires very expensive devices for carrying it out. Furthermore, said method does not allow to differentiate between surface flaws and dust or water marks on the surface of the component.
German Patent Application No. 3620108 describes a method according to which the optical component to be tested is rotated on its axis and is illuminated by a light beam which is displaced along the diameter of the component at a frequency greater than the angular rotation speed of the component, whereby a spoke-shaped pattern is generated. This pattern moves once through the component when the same is turned by 360%, and in this way uniformly illuminates each point of its volume with light in approximately the same state of focus. The radiation, the normal diffusion of which is disturbed by any possible flaws in the surfaces of or inside the component, is detected in the post-installed testing system and only the light diffracted by flaws in the component Is used to produce the image. A device operating in this way is expensive and this method does not differentiate between dust or water marks and flaws of a component.
DE-OS 3237511 discloses placing the optical components to be tested in the optical path of a television camera and displaying through the component a test pattern on the camera. The disturbances caused by flaws in the component produce a video signal which deviates from a control signal that is not influenced by the component. The deviations between the control signal and the actual signal received permit to identify the flaws. By this method, however, smaller flaws, such as those resulting from scratching, smears or hairline cracks, cannot be detected, and it is impossible to differentiate dust or water marks from surface flaws.
DE-OS 3011014 attempts to increase the sensitivity of the aforesaid testing procedure by suggesting that the component to be tested be illuminated completely, a television image be produced, and flaws be deduced by line-for-line analysis of the video signal. Such a procedure is complicated, expensive, not sufficiently accurate, and does not allow to differentiate dust and water marks from flaws or occlusions.
U.S. Pat. No. 3,988,068 discloses a method for detecting cosmetic defects in lenses, where in order to distinguish between actual defects and surface dust, it is suggested to direct a light into the lens through its peripheral edges so that the light will impinge upon the front surface at angles more than critical angle to undergo one total internal reflection within the lens and leave the lens though its edge, and to detect a light scattered by any of the defects. It is mentioned in the patent that for this purpose the light sources should be aligned relative to a front surface of the lens, which alignment may be "easily . . . accomplished with slight amount of experimentation". However, there is not a single hint in the patent how to provide such alignment or according to which requirements it should be accomplished. Neither it is mentioned how to avoid emerging of light, which travels within the lens in a non-controllable way, through inspected surface and how to avoid the illumination of the non-finished surface by the non-controlled illumination light. Moreover, the disclosed method is not effective as it clearly cannot provide for sufficient detection capability seeing that the light from each illumination source illuminates each defect under an extremely narrow range of angles thus decreasing a possibility of the scattered light to reach the detection means and consequently its detectability. Furthermore, the illumination of the inspected surface and bulk of the lens is not homogeneous influencing the sensitivity of the method. In addition, it is very important in this method for the edge of the lens to be of good quality and this requirement also restricts applicability of the method.